While it's not difficult to use a good RNG it's quite likely that none of these machines have been significantly updated since they were first designed and when

While it's not difficult to use a good RNG it's quite likely that none of these machines have been significantly updated since they were first designed and when they were first designed they probably weren't thinking about attack vectors. The exploit isn't being able to crack incredibly well designed RNGs, the exploit is being able to crack crap RNG based machines without the casino or machine manufacturer noticing. Slot machines are a huge industry and no one thinks of them as an attack vector, so these guys can probably milk quite a bit of cash out of them without anyone ever noticing. Meanwhile the cost of fixing these machines is high - they're not going to be set up for OTA updates, so the manufacturer now has to spend probably 2 days changing the RNG, and the next 6 months delivering updates to all their customers and explaining to them that their slot machines were probably costing them several hundred thousand dollars.